1. Field of the Invention
This invention relates to transfer type image forming technology and in particular to a charging device for uniformly charging an image forming member as the first step of an image forming process.
2. Description of the Prior Art
A corona charging device for charging an image forming member which includes a photoconductive layer or a dielectric layer for forming an image thereon is well known in the art. For example, in a transfer type electrophotographic or electrostatic image forming system, an image forming member including a photoconductive or dielectric layer for forming an image thereon is first charged uniformly to a desired polarity by means of a D.C. corona charging device, and then the charges thus deposited on the image forming member are selectively dissipated in accordance with an image information to form an electrostatic latent image. Then toner is applied to the image forming member to convert the latent image into a visible toner image, which is then transferred to a transfer medium such as recording paper. On the other hand, after transfer, the image forming member is subjected to discharging and cleaning steps thereby removing residual charges and toner particles to provide the image forming member ready for the next cycle of image forming operation.
When uniform charging of the image forming member is to be carried out by a D.C. corona charging device, typically a scorotron charger, in the above-described image forming process, it is difficult to charge the image forming member to the saturation level primarily because of a relatively short period of time for charging. This is particularly true for the image forming member including a conductive support and a dielectric layer overlying the conductive support. As a result, the level of the surface potential of the image forming member after uniform charging varies from one cycle of operation to another in a repetitive operation. This is disadvantageous because resultant images are not uniform. Thus, in order to maintain the resultant images uniform in quality, it is necessary to control the bias voltage applied at the time of formation of a latent image and/or developing the latent image optimally, which is also disadvantageous since such a control operation is difficult to carry out. Under such circumstances, if use is made of corona transfer technology, the surface potential of the image forming member is affected additionally thereby making it still more difficult to maintain the surface potential of the image forming member substantially at the same level each time after uniform charging.
FIG. 1 shows the charging characteristics of a scorotron charger with the grid voltage V.sub.g as a parameter. The abscissa is taken for the discharging time in seconds and the ordinate is taken for the potential of the image forming member in volts. The characteristics shown in FIG. 1 are for the case in which the corona voltage is equal to 8,100 V and the onset voltage is equal to 4,100 V. As shown in FIG. 1, the rising time for the image forming member to reach the saturation level is relatively long; for that matter, the time constant in charging is rather large. For this reason, in order to carry out uniform charging at a high speed, which is usually required in a high speed image forming operation, the discharging step for removing residual charges from the image forming member is required prior to the step of uniform charging. In the case of absence of such a discharging step in the prior art, the surface potential of the image forming member tends to vary from one cycle of operation from another as shown in FIG. 2. As shown, the level of the surface potential differs in each cycle, T.sub.1, T.sub.2 and T.sub.3 of operation, and, in some cases, it gradually increases in the positive direction as shown by the solid line, or in some cases, it gradually increases in the negative direction as shown by the dotted line. In FIG. 2, T.sub.s corresponds to a charging period by a scorotron charger; T.sub.r to a recording and developing period; and T.sub.t to a transferring period in one cycle of operation.
In the case where the image forming member includes a photoconductive layer, the image forming member may be discharged relatively easily and the surface potential may be set to 0 level by irradiation with uniform light; however, depending upon the material of the photoconductive layer, the service life of the image forming member could be significantly reduced due to such irradiation with uniform light. On the other hand, in the case where the image forming member includes a dielectric layer, it cannot be discharged by irradiation with uniform light, so that discharging in this case is usually carried out by A.C. corona discharge. In this case, however, it is rather difficult to set the surface potential at zero level by A.C. corona discharge, and, therefore, the gradual change in surface potential can only be slightly reduced as compared with the case shown in FIG. 2.